Motors, Part 13

Suppose you diligently follow the requirements of 430.32(A)(1) and (B)(1) when sizing the overload device for an industrial motor. But many times when starting the motor, the overload blows.

You have three possible solutions to choose from. The NEC provides one: you can simply use the next size up overload [430.32(C)]. If that doesn’t work, you can try the next size up and keep going until you’ve got it.

The limitation is that the trip current of the motor overload device can’t exceed 130% of the motor full load current rating. But if the motor is marked with service factor of 1.15 (or greater) or marked with a temperature rise of 40°C (or less), you can go up to 140%.

The advantages of this solution:

Least expensive in terms of initial cost.

Easiest and quickest to implement.

The disadvantage may be that it’s horribly expensive in terms of total cost of ownership. If you are getting that much inrush, there are negative effects on the rest of the distribution system.

With a large enough motor, you’ll see system-wide voltage sags / current spikes followed by system-wide voltage spikes. By “system,” we mean everything connected to the same service. This damages conductor insulation; you can verify this by comparing insulation resistance readings year to year. It also causes damage to connected equipment. Smaller motors, for example, will get pinholes in the insulation of their windings.